Department of Psychology, Program in Behavioral and Systems Neuroscience, Rutgers University.

Abstract

Early in the 20th century, individual differences were a central focus of psychologists. By the end of that century, studies of individual differences had become far less common, and attention to these differences played little role in the development of contemporary theory. To illustrate the important role of individual differences, here we consider variations in intelligence as a compelling example. General intelligence (g) has now been demonstrated in at least 2 distinct genera: primates (including humans, chimpanzees, bonobos, and tamarins) and rodents (mice and rats). The expression of general intelligence varies widely across individuals within a species; these variations have tremendous functional consequence, and are attributable to interactions of genes and environment. Here we provide evidence for these assertions, describe the processes that contribute to variations in general intelligence, as well as the methods that underlie the analysis of individual differences. We conclude by describing why consideration of individual differences is critical to our understanding of learning, cognition, and behavior, and illustrate how attention to individual differences can contribute to more effective administration of therapeutic strategies for psychological disorders. (PsycINFO Database Record.

The hierarchical model of intelligence, where a general ability co-exists with specialized abilities. At Level 1, people differ in their performance on specific tests that are representative of various cognitive domains (or “modules”). Within a domain, performance measures are highly correlated. Given this high correlation, a latent trait at the domain level (Level 2) would be identified through factor analysis. Level 3 represents a general influence that impacts performance regardless of domain, i.e., people who do well in one domain also tend to do well in the other domains. It is this later observation that requires the existence of a general cognitive latent trait, or g (general intelligence). Nevertheless, correlations between tasks within a domain are typically higher than the correlations between tasks that reside in different domains, requiring the existence of domain-specific abilities.Note: The domains described in this figure are merely illustrative, as theoreticians differ as to the actual composition and number of domains that underlie human cognitive functions. The “? Domain” is intended to represent this uncertainty.